EYES OF CHOLOGASTER PAPILLIFERUS. 115 



cells here form a distinct row in section, while the rest for the most part are 

 irregularly placed. The difference in size is especially noticeable near the entrance 

 of the optic nerve. The nuclei are mostly spherical. A few nuclei are found 

 more elongate and with their longer axis at right angles to the retina (Muller's 

 fiber nuclei). The largest spherical nuclei measure 5 /*. in diameter. The inner 

 granular layer varies in thickness and contains few cells. 



The ganglionic layer consists of a single layer of nuclei, rather irregularly placed. 

 The nuclei measure 6 /a in diameter. For reasons explained in a previous para- 

 graph a distinct nerve-fiber layer is not present. A thin nucleate membrane, the 

 hyaloid membrane, containing the blood-vessels, lies directly on the ganglionic layer 

 (%• 35 c). _ 



It is quite evident from the foregoing that the retina is very much simplified as 

 compared with that of Zygonectes. The point of greatest degeneration lies between 

 the outer nuclear and inner reticular layers. The horizontal nuclei are all but 

 entirely eliminated. The bipolar cells are, in the adult, reduced to two layers of 

 nuclei, and the spongioblasts are reduced to a single layer of cells. Even this dis- 

 tinction and differentiation is only seen in the largest individuals. Twin cones are 

 abundant and apparently not lacking in number and structure, but are arranged 

 in a different manner. Rods are much fewer in number than in either Coregonus 

 or Zygonectes. 



The chief difference between the youngest and oldest specimens of papilliferus 

 examined lies in the thickness of the pigmented layer and the outer nucleated and 

 the outer granular layers. The relative thickness of the pigmented layer increases 

 very much with age. 



The irideal region needs a few words since its structure helps to explain certain 

 conditions in the blind fishes. The epithelial part is composed of two layers of 

 cubical cells, of which the outer are the larger. The 

 outer cells are normally filled with pigment to such O& c\ 



an extent that their outlines can not be made out, the QOq q 



inner cells are free from pigment. The outer layer _0 0>0 O Q ^q 

 passes directly over into the pigmented layer of the ^ C\ ^O/S'Gu 

 retina. Where the inner layer of the iris merges into the & ^ 0<?<\& 

 inner layers of the retina it is composed of a group of oQV 0r\ c?^(v f) 

 cells with elongated nuclei (fig. 36/, nl. I.). The uveal £k ^1 Vp (y 

 part of the iris is composed of a thin layer of cells A 



with irregular nuclei, and the pigment cells of this k.g. 37 . Nuclei of Epithelial Layer of 



° . ' r ° ..... . Lens of Chologaster. 



layer are much thinner than the epithelial pigment. 



The ligamentum ciliary does not contain many muscle fibers, but is abundantly 

 supplied with granular nuclei. The things of greatest importance are these granu- 

 lar nuclei, the epithelial pigment and the oval nuclei at the ora serrata. As com- 

 pared with the same region in other fishes the shortness of the section of the iris is 

 at once striking (fig. 36 g). The absence of ciliary muscles and the insignificance 

 of Decemet's membrane are also notable. 



The lens offers no peculiarities. The shape of its epithelial nuclei may be seen 

 in figure 37. 



